The invention relates to an implement linkage draft force sensing arrangement. That is an arrangement for measuring the tensile and compressive forces applied to the implement linkage during use. It is particularly suitable for use on the top-link of a tractor three-point linkage but is more widely applicable.
Implements are almost universally connected to the rear of agricultural tractors by means of a three-point linkage which is raised and lowered hydraulically. Some form of draft force sensing is included in the system in order that the linkage can be controlled in response to the measured draft force. The draft force may be sensed either on both lower-links of the three-point linkage or on the single top-link. The simplest systems use mechanical connections to control the hydraulics, however, in recent years these have been largely superseded by electrical sensors and control electronics.
1. Description of the Prior Art
It is well known in the industry to measure the draft force using two electrical sensing pins, one for each lower link of a three-point linkage. The forces measured by the two pins vary widely as lateral forces are applied to the implement and therefore the linkage is controlled in response to the average. Such systems, however, suffer from the disadvantage that they use two sensing pins, as these are expensive, and additional processing is required.
As mentioned above, it is also known in the industry to measure draft force applied to the single top-link of the three-point linkage. It has been proposed to do so directly by using a sensing pin as a spindle to articulate the top-link on the tractor. This suffers from the disadvantages that it is difficult to demount the upper arm, the pin experiences high wear and is exposed to high forces requiring it to be of large diameter. An alternative top-link system is described in French Patent No. 89 12181 (publication no. 2 636 497). The top-link is articulated on the tractor by a rocker which is attached to the tractor at both ends by means of spindles forming bearing axes. One of the spindles takes the form of a well known electrical draft force sensing pin. However, this system still suffers from the following disadvantage. Practical engineering tolerances mean that the holes in the rocker will not be perfectly aligned with the corresponding holes in the tractor frame, so that static forces on the pins are unpredictable.